It is known to provide brewing units with valve assemblies for regulating the outflow of the coffee beverage delivered from the brewing chamber.
WO 02/091889, in the name of DE LONGHI, discloses a filter-holding cup for a coffee machine. The filter-holding cup coupled to the related machine defines a brewing chamber. The cup comprises a spring biased valve arranged downstream from the filter. Most of the time the valve body is kept in a closing position by the spring. Only during operation of the machine, the coffee beverage exiting the filter impinges the valve body against the bias force of the spring, thereby causing the valve body to move axially in an opening position.
In the aforesaid or similar solutions, the coffee flow valve, which is arranged downstream from the brewing chamber, is wetted by the coffee beverage exiting the brewing chamber at each dispensing cycle of the machine. Residues of the coffee beverage have adhesive properties and build up on the valve seat and the valve body surfaces, acting as a “glue” and eventually causing jamming of the valve body within its seat.
This problem occurs in particular in brewing units wherein the valve body is made of (or covered with) a plastic material, for instance silicone or a similar thermoplastic material. Such material, when soiled by the coffee delivered from the brewing chamber, easily adheres to the seat, thereby keeping the valve assembly in a closed position.
In order to avoid jamming of the valve assembly, the valve body is often removed by the final user of the coffee machine or the maintenance operator. In other words, prior art coffee machines affected by the aforesaid problem are often tampered for removal of the valve body. Therefore, operation of the coffee machines changes with respect to the parameters preset by the manufacturer and the quality of the delivered coffee is lowered. In fact “espresso” coffee is usually prepared by brewing in a chamber substantially closed, while “americano” coffee is prepared by brewing in a chamber having its exit open.
U.S. Pat. No. 6,382,083, in the name of SCHMED, discloses a brewing unit for coffee machines provided with a valve assembly inserted between the brewing chamber and the beverage outlet in order to control the flow rate of the coffee beverage. The valve assembly comprises a valve body member movable against the force of a spring between a first position, wherein the valve body does not close the seat but leaves a minimum gap, and a second position, wherein the valve body still does not close the seat and leaves the maximum gap. The spring is set to keep the valve body member in the open position. By adjusting the bias force of the spring, the user changes the extension of the gap between the valve body and the related seat, this causing changes in the amount of coffee to be brewed per time unit. Italian patent IT 1142041, in the name of GROSSI, discloses a brewing unit wherein a valve is positioned in a passage provided in the filter element for regulating the outflow of the coffee beverage prepared in the brewing chamber. This valve has a valve body which is axially movable and has size and the shape such that, during preparation of the coffee beverage, it does not close the outlet passage of the filter element but leaves an annular gap. Presence of such annulus gap should prevent malfunctioning of the brewing unit due to clogging of the filter element.
Brewing units similar to those disclosed in U.S. Pat. No. 6,382,083 and IT 1142041, do not allow for closing the valve and relevant pressure build-up within the brewing chamber to e.g. prepare an espresso coffee.
Thus, there is the need for a brewing unit which allows for closing of the valve assembly downstream from the brewing chamber and, at the same time, prevents jamming of the valve body.